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"We simply went into supplement retailers, or bought online, supplements that were being marketed for athletes as 'performance-enhancing' product," said study lead author, Professor Alison Heather of Otago University's Department of Physiology.

Sports supplements, as well as other forms of nutritional supplements, were popular with athletes - particularly body-builders.

"Some members of the general population also take them in the hope they might improve body image or vitality."

But the sports supplement market was not well regulated and manufacturers can market their products with unsubstantiated claims, Heather said.

While supplements could contain ingredients that may have useful properties, poor manufacturing practices and adulteration could add compounds that were banned for use in sports, yet not included on the label.

"Sports supplements contaminated or adulterated with androgens are a major concern because, for the unsuspecting consumer, they pose a potential health risk, while for an athlete it could result in a positive doping test."

In 2015, a report showed 95 per cent of elite New Zealand athletes consumed sports supplements, which followed a report a year earlier that 70 per cent of New Zealand first XV rugby players regularly took supplements.

"Our research shows that there is a real risk for health and doping violations that athletes must consider when taking sports supplements, even those sold over the counter."

Supplements had been used more and more in recent years, from elite athletes down to weekend warriors, he said.

"However unfortunately athletes do not always know what goes into the supplement, and this research confirms that."

Buying products off the internet only heightened the risk, Paterson said.

"We would like athletes to really think about whether or not supplements are necessary in their sport, and whether the risk of taking one, to your health and of a two-year ban for being a doper, is worthwhile."

Medsafe general manager Chris James said his agency was now interested in finding out more about the products that tested positive.

"Medsafe is aware that there is the potential for sports supplement products to be non-compliant with the legislation and will take appropriate action, as necessary," he said.

James also noted consumers should be aware there was no pre-market approval for dietary supplements and other similar products.

Instead, obligations were placed on the suppliers to ensure that their products comply with laws around medicines and dietary supplements.